Former French President Nicolas Sarkozy has reportedly been charged with corruption and influence peddling in a criminal case.

Agence France-Presse reported the charges came after Sarkozy was held for questioning for 15 hours over suspicions he used his influence to secure leaked details of an inquiry into alleged irregularities in his 2007 election campaign.

It was the first time a former French head of state has been held in police custody and is the latest blow to Sarkozy's hopes of a comeback after his 2012 election defeat by Socialist rival Francois Hollande. The conservative politician denies all wrongdoing in a string of investigations involving him.

Sarkozy arrived early on Tuesday (local time) to be quizzed by police investigators at their offices in Nanterre, west of Paris. He spent all day and evening in police custody, but about 11.40pm he was seen by a Reuters journalist arriving at a civil court in Paris, where he was to be presented to judges.

Under the French legal system, being sent before the court would be the next step in the possible opening of a formal investigation against a suspect - a step that often, but not always, leads to trial.

A few hours earlier, Sarkozy's attorney and a judge involved in the case were put under formal investigation on suspicion of influence peddling, their attorneys said.

Placing a suspect under formal investigation means there exists "serious or consistent evidence" pointing to probable implication of a suspect in a crime. Investigating magistrates, rather than the police, then conduct their own probe.

Influence-peddling can be punished by up to five years in prison and a fine of 500,000 euros (NZ$780,000). Sarkozy lost presidential immunity from legal prosecution a month after he left office in June 2012.

Allies rushed to support Sarkozy.

"Never has any former president been the victim of such treatment, such an outburst of hatred," Christian Estrosi, the mayor of the southern city of Nice and a close Sarkozy ally, said on his Twitter account.

Government spokesman Stephane Le Foll, however, said Sarkozy was "subject to justice like everyone else" and told i<Tele television that the justice system needed to "go all the way."

"STASI" TACTICS?

The probe is one of six legal cases involving Sarkozy either directly or indirectly, including more recent allegations of irregularities in his unsuccessful 2012 election campaign.

The current questioning relates to suspicions he used his influence to get information on an investigation into funding irregularities in his victorious 2007 election campaign.

One cloud was lifted off Sarkozy's future last October when a court dropped inquiries into whether he had exploited the mental frailty of France's richest woman, L'Oreal heiress Liliane Bettencourt, to fund that campaign.

But as investigators used phone-taps to examine separate allegations that late Libyan leader Muammar Gaddafi funded the same campaign, they began to suspect he had kept tabs on the Bettencourt case through a network of informants.

Those suspicions finally prompted the formal launching of yet another investigation into influence-peddling in February. Sarkozy has likened the magistrates behind the phone-tapping to the "Stasi" police of former communist East Germany.

Sarkozy, 59, retired after his defeat by Hollande but has continued to snipe both at the Socialist president and rivals inside his own conservative camp with messages carefully placed in local media by his political entourage.

Sarkozy remains the favorite of conservatives to challenge Hollande but he is widely detested by left-wingers and his abrasive style alienated many middle-of-the-road voters.

Asked about his future at a closed-door event at France's parliament last week, Sarkozy said he was still "in a period of reflection" but indicated he would make up his mind in coming months whether to seek the 2017 ticket of his UMP party.

Yet a growing number of voices in the UMP have been arguing he is too much of a liability to run for them as president.

The calls grew louder last week as a separate funding scandal over his 2012 campaign escalated.

The former deputy director of Sarkozy's campaign, Jerome Lavrilleux, said four UMP members besides himself agreed to use false accounting to cover rising campaign expenses that had surpassed a legal limit. Lavrilleux said Sarkozy himself was not aware of the accounting irregularities.